boxplot_data.rb

require 'gsl'

a = [0,1,2,3,4,8]
v = GSL::Vector.alloc(a).sort
q1 = v.quantile_from_sorted_data(0.25)
q3 = v.quantile_from_sorted_data(0.75)
iqr = q3 - q1
tw = q3 + iqr * 1.5
bw = q1 - iqr * 1.5
to = a.filter { |x| x > tw }
bo = a.filter { |x| x < bw }

# - https://www.gnu.org/software/gsl/doc/html/statistics.html#median-and-percentiles
# - https://github.com/ampl/gsl/blob/dee1063dd57ec2841981b010eb732901b452c30a/statistics/quantiles_source.c
# - https://muse.union.edu/dvorakt/what-drives-the-length-of-whiskers-in-a-box-plot/